Handover mechanisms in next generation heterogeneous wireless ...

TRUST ASSISTED HANDOVER ALGORITHM FOR RELIABLE HANDOVER
the moment a mobile user loses its connection with the old POA (oPOA) to the time it
receives the first packet from the new POA (nPOA). To analyse handover delay, a
system model is developed.
In a typical “break-before-make” handover scenario of the interconnected WLAN-
UMTS network, handover delay comprises of the following major components:
Movement Detection (MD): Movement detection delay ( t MD ) is the period of time
taken on deciding whether a mobile user has moved to a different network. Due to the
asymmetry of move-in and move-out handover scenarios [89] in the interconnected
WLAN-UMTS network, WLAN�UMTS and UMTS�WLAN handover mean
different movement detection latencies. In Mobile IPv4 specification [21], two
algorithms have been specified with regard to movement detection. In the first method,
a mobile user checks agent advertisement lifetime (ADF) periodically. Upon the
expiration of ADF, it assumes that it has lost contact with the agent router. In the second
approach, the network prefix of the advertisements from the nPOA is compared with the
network prefix of the oPOA. A change in network prefix would imply that the mobile
user may have moved to another subnet. In this study, it is assumed that the first method
is applied to WLAN�UMTS handover and handover between homogeneous networks.
While, for UMTS�WLAN handover, t MD is equal to 0 because discovering a WLAN
nPOA would not cause transmission interruption if a second interface is employed.
Network Selection (NS): Network selection delay ( t ) is denoted as the time required
for network solicitation (L2+L3) (� ) in absence of the valid router advertisements from
the nPOA, plus handover decision making time ( � ) at mobile terminals. Mobile
terminals with multiple network interfaces [92] can be enabled to conduct network
solicitation using a standby interface, while carrying data traffic on the primary
interface simultaneously. To simplify the analysis, it is assumed that handover occurs
only between heterogeneous wireless networks. When multiple interfaces are
simultaneously used [92], network solicitation can be completed preliminarily ( � � 0 ).
While, � is often a constant determined by the computing power of mobile terminals.
Therefore, t
is believed to be able to remain unchanged because handover decision
NS
can be made independent of network discovery.
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NS